The chemical industry performs numerous equilibrium controlled reactions to manufacture a wide range of chemical raw materials, intermediates and products. Product yield obtained in such equilibrium controlled reactions is typically limited by the thermodynamic equilibrium of the reaction. Therefore, such reactions are typically operated at an elevated temperature for endothermic reactions or at a reduced temperature for exothermic reactions in order to shift equilibrium toward the product direction. Thus, the chemical industry has been searching for improved processes for operating equilibrium controlled reactions at reduced temperatures for endothermic reactions wherein product yield is not substantially diminished due to unfavorable thermodynamic equilibrium constants.
Representative equilibrium controlled reactions include methane and hydrocarbon steam reforming reactions which are used to manufacture hydrogen or synthesis gas, the water gas shift reaction for converting CO to CO.sub.2, as well as the reverse water gas shift reaction for converting CO.sub.2 to CO. Some of these reactions are typically carried out at relatively high temperatures to shift the equilibrium toward the product direction as well as to obtain relatively faster reaction kinetics. Significant efforts have been described in the literature to improve reaction kinetics by identifying new catalysts and by controlling process operating conditions. Additionally, the concept of removing a product from a reaction zone to increase product conversion is well known.
Representative processes for operating equilibrium controlled reactions include an article by Vaporciyan and Kadlec (AIChE Journal, Vol. 33, No. 8, August 1987) which discloses a unit operation comprising a rapid pressure swing cycle in a catalytic-adsorbent bed to effect both continuous gas-phase reaction and separation. The hybrid device combines features of a cyclic-steady-state pressure swing adsorber with those of a flow-forced catalytic reactor.
Westerterp and coworkers (Hydrocarbon Processing) p. 69 (November 1988) disclose two process schemes for improving conversion of hydrogen and carbon monoxide to methanol. The first embodiment employs a Gas-Solid-Solid Trickle Flow Reactor (GSSTFR) wherein a solid adsorbent is trickled through a packed bed reactor to remove methanol from the reaction zone which results in increased production of methanol. The adsorbent saturated with methanol is collected On a continuous basis using multiple storage tanks wherein the methanol is desorbed by reducing the pressure. The second embodiment employs a Reactor System with Interstage Product Removal (RSIPR) wherein methanol is synthesized in several stages and removed utilizing a liquid solvent. High conversion of methanol per pass is achieved in a series of adiabatic or isothermal fixed bed reactors. Product is selectively removed in absorbers situated between the respective reactor stages.
J. Berty and coworkers (Chemtech, p. 624 October 1990) disclose a Solvent Methanol Process (SMP) wherein methanol is prepared using a reactor consisting of a fixed catalyst bed into which a stream of high boiling inert solvent is introduced concurrent with the synthesis gas stream. The solvent selectively absorbs methanol as soon as it is formed over the catalyst bed causing the methanol activity to remain low and to shift the equilibrium toward the product. The product-rich solvent is depressurized causing methanol to be released and lean solvent is recirculated to the reactor.
E. Kikuchi and coworkers (Stud. Surf. Sci. Catal., 61 (Nat. Gas Convers.), 509 (1991) disclose a process for producing hydrogen wherein a palladium membrane reactor is utilized to operate a methane steam reforming reaction. The reactor, formed by supporting a thin palladium-silver alloy membrane onto porous alumina ceramics, possesses infinite selectivity of hydrogen over other reactant and product gases and increases the conversion of hydrogen at high temperature in the methane-steam reforming operation. The investigators demonstrate that conversion to product is correlated to reduction in hydrogen concentration by permeation through the reactor. Conversion of reactants to products at low temperatures could be limited in such reactors because the driving force for hydrogen permeation decreases at low permeant partial pressures, thus resulting in a lower limit to the hydrogen partial pressure which can be achieved in the reactor.
Kirkby and Morgan (The 1991 Icheme Research Event) present a mathematical model which is stated to demonstrate the general advantages of simultaneously conducting reaction and separation steps wherein the residence time of the components can be manipulated independently within the reactor. The investigators postulate that the disclosed pressure swing reaction system is expected to be superior to conventional thermal cracking systems because the use of a suitably selected mixture of catalyst and adsorbent may result in lower reaction temperatures and may reduce the rate at which coking products are formed. Furthermore, the simultaneous separation of a product component from the reactor may permit conversions far in excess of the normal equilibrium values.
Goto and coworkers (Chemical Engineering Essays, Vol. 19, No. 6 (1993) disclose pressure swing adsorption processes for dehydrogenating cyclohexane using a hydrogen occlusion alloy. The investigators state that continuous process operation can be accomplished by staggering the reaction and adsorption phases in a plurality of reactors. The reactor bed can be regenerated by pressure swing adsorption. The pressure swing adsorption reactor is operated continuously by adsorbing the product within the reactor immediately as it is formed and desorbing the adsorbed product by applying vacuum and by purging with helium.
Prior art processes for conducting simultaneous reaction and adsorption steps have not achieved commercial success because product flow rates do not remain sufficiently constant and the desired products are present in unacceptably low concentrations with respect to the undesired reaction products, unreacted feedstock and purge fluids. Industry is searching for a process for operating equilibrium controlled reactions which can be operated in continuous mode at reduced reaction temperatures wherein a reaction product can be produced in substantially pure form at high conversion, under relatively constant flow rate and at feedstock pressure.